Visual display devices

ABSTRACT

A visual display device includes a matrix of direct current gas discharge cells in a block of electrically insulating material. For each row and column of the matrix, all cells in that row or column are interconnected by a row or column conductor so that energisation of appropriate row and column conductors strikes the defined cells in the matrix. Scanning means is also provided by means of which each column conductor is energised in turn without the need for a driver for each column. Information may be written on the display device by energisation of the appropriate row conductors as each column conductor is energised, the information display remaining after the column conductor has subsequently been de-energised.

United States Patent 1 Walters [5 1 VISUAL DISPLAY DEVICES [75] Inventor: Frank Walters, Bury, England [73] Assignee: Ferranti Limited, l-lollinwood,

Lancashire, England [22] Filed: Aug. 10, 1971 21 Appl. No.: 170,474

[ Jan. 22, 1974 Primary Examiner-H. K. Saalbach Assistant Examiner-Lawrence J. Dahl Attorney, Agent, or FirmGordon W. Daisley; Cameron, Kerkam, Sutton, Stowell & Stowell [5 7] ABSTRACT A visual display device includes a matrix of direct current gas discharge cells in a block of electrically insulating material. For each row and column of the matrix, all cells in that row or column are interconnected by a row or column conductor so that energisation of appropriate row and column conductors strikes the defined cells in the matrix. Scanning means is also provided by means of which each column conductor is energised in turn without the need for a driver for each column. Information may be written on the display device by energisation of the appropriate row conductors as each column conductor is energised, the information display remaining after the column conductor has subsequently been de-energised.

8 Claims, 5 Drawing Figures 1 VISUAL DISPLAY DEVICES This invention relates to visual display devices, and particularly to such devices comprising an array of gasfilled direct-current cold-cathode dischargecells each of which may be struck or extinguished to produce a display of the required form.

Visual display devices are known which incorporate an array of direct-current discharge cells formed by one or more gas-filled cavities in a block of an electrically-insulating material such as glass or ceramic. A set of conductors is formed on a further block which is then sealed to the first block so that each conductor forms, or is connected to, one electrode of each of a number of discharge cells. Each discharge cell is also provided with another electrode and a series impedance connected between that electrode and one of a second set of conductors. Such a visual display device is hereinafter referred to as of the type described.

One of the limitations of visual display devices of the type described has been the relative complexity of the circuitry necessary to operate the device. This is because the two conductors which uniquely define any one cell both have to be energised to cause that cell to strike. D.C. -excited panels have been made with simpler circuitry by scanning the columns of cells with a three-phase scanning waveform, but these have the disadvantage that the display has to be continually rewritten.

It is an object of the invention to provide a visual display device of the type described which enables the drive circuitry to be considerably simplified.

According to the present invention there is provided a visual display device which includes a plurality of dis charge cells comprising a block of electrically insulating material containing a plurality of gas-filled cavities arranged in a two coordinate array, a first set of electrical conductors each interconnecting one end of each cavity associated with a particular value of one coordinate, a plurality of electrically resistive elements each associated with a cavity and having a part located at the other end thereof, and a second set of electrical conductors each interconnecting the resistive elements of each cavity associated with a particular value of the other coordinate, the device also including a glow transfer device, one electrode of each of the pairs of electrodes of the glow transfer device being connected to a separate conductor of one of said sets, and means for sequentially energising the other electrodes of each of the pairs so as effectively to scan the conductors of said one of said sets.

In this specification the term glow transfer device is used to define a gas-filled discharge tube in which a discharge is caused to move from one pair of electrodes to another by the mechanism known as glow transfer.

The invention will now be described with reference to the accompanying drawings, in which:

FIG. I is a schematic diagram of part of a display device with integral glow transfer device;

FIG. 2 is a schematic diagram of part of the glow transfer device;

FIG. 3 illustrates the necessary control signal waveforms; and

FIGS. 4 and 5 are sectional side and end elevation views of part of a display device with integral glow transfer device.

Referring now to FIG. I, this shows in schematic form, part of a rectangular matrix of discharge cells.

The discharge cells are arranged in rows and columns and suitably interconnected by row conductors RC1, RC2 etc, and column conductors CCl, CC2 etc. Each discharge cell is represented by a discharge tube symbol and a series resistance, connected between one row conductor and one column conductor. Each row conductor is connected to a drive circuit RDI, RD2 etc., whilst each column conductor is connected through a resistor R1 to a suitable potential, shown in FIG. 1 as earth potential.

The device as so far described is a conventional form of display device. The device illustrated in FIG. 1, however, also includes an integral glow transfer tube having a plurality of pairs of electrodes. One electrode of each pair is formed by the column conductors CC already mentioned. The other electrode of each pair is formed by a transfer conductor TC located close to each column conductor as shown. Each transfer conductor is connected to'a pulse circuit 10. The pulse circuit 10 is a three-phase pulse generator producing a pulse at each of three outputs X, Y and Z in turn before repeating the sequence. An additional output W provides a trigger pulse after a specified number of cycles of the threephase signal, the three-phase signal being delayed for the duration of the pulse. The waveforms at the outputs W,X, Y and Z of the pulse generator 10 are shown in FIG. 3. Every third transfer conductor is commoned to give three connections which are made to the X, Y and Z outputs of the pulse circuit 10.

In addition to the glow transfer electrodes formed by the column conductors CC and the transfer conductors TC, a further pair of electrodes is provided, denoted by conductors RS]. and RS2. Conductor RS1 is connected through a further resistor R1 to earth potential, whilst conductor RS2 is connected to the output W of the pulse circuit. The purpose of these further electrodes will be described later.

FIG. 2 illustrates in schematic form the arrangement of the electrodes in the integral glow transfer tube. The individual discharge cells are not shown, but these have their cathodes connected to, or formed by, the column conductors CC. As shown in FIG. 2, the pairs of glow transfer electrodes formed by the cathode conductors and the transfer conductors are arranged on opposite sides of a gas-filled cavity 11. The electrical connections are as described above.

The operation of the glow transfer device will now be described with reference to FIGS. 2 and 3.

Consider initially the case where no discharge exists in the glow transfer tube. Each output of the pulse circuit 10 is held at a negative potential -V1 approximately equal to the extinction voltage of the glow transfer discharge. The pulse circuit 10 applies via its output W to the conductor RS2 a negative potential V3 of sufficient magnitude to break down the gap between conductors RS1 and RS2, and initiate a discharge between them. After a predetermined time interval the output W from the pulse circuit will revert to the value VI, and output X will have applied to it a negative potential V2. This is sufficient to transfer the discharge to TCl and CCI whilst ions produced between conductors RS1 and RS2 are still present in the vicinity, though it is not sufficient to strike a discharge between conductors CC and TC4, to which the output X is also applied, in the absence of such ions. The glow discharge will thus transfer from the glow initiating electrodes RS1, RS2 to the adjacent pair of electrodes CCl, TC 1. When output X of the pulse circuit reverts to V1 and output Y is taken to V2, the discharge will transfer to the gap between electrodes CC2 and TC2. Similarly, when the output Y reverts to V1 and the output Z is taken to V2, the discharge moves to electrodes CC3 and TC3. The-output X is then reduced to V2 again but this time the discharge moves from electrodes CC3 and TC3 to electrodes CC4 and TC4 in preference to a fresh discharge striking between electrodes CCl and TCl or indeed between electrodes CC7 and TC7. Hence the glow discharge is caused to move along the glow transfer device by sequential energisation of the outputs X, Y and Z of the pulse circuit. When the end of the glow transfer device is reached, it is again necessary to energise output W of the pulse circuit to re-establish a discharge at the beginning of the cycle. For this reason, the electrodes RS1 and RS2 are also known as reset electrodes. This mechanism is a conventional glow transfer mechanism, and the pulse circuit 10 is required to provide only a sequence of pulses on four outputs. The shape of the pulses may be varied, as may the degree of overlap between successive pulses applied to adjacent pairs of electrodes.

The effect of the scanning of the glow transfer device on the matrix of discharge cells will now be described. lt should be remembered that each discharge cell is connected to one or other of the column conductors CC, and that each cell connected at one end to a particular column conductor is connected at its other end to a different row conductor RC and row drive circuit RD.

The formation of a discharge between the reset electrodes RS1 and RS2 has no effect on the discharge cells. However, when the discharge in the glow transfer device forms between electrodes CCl and TCl, the discharge current flows through the appropriate resistor R1. This causes the voltage of conductor CCl to fall to a value determined by the value of R1 and the current through the transfer discharge. Hence a negativegoing pulse is applied to one end of each discharge cell connected to column conductor CCl. However, the magnitude of the pulse is arranged to be considerably less than the voltage necessary to cause a discharge cell to strike. Each row drive circuit RD is arranged to produce positive-going pulses when required in synchronism with the negative-going pulses occurring on the column conductors. This positive-going pulse is also of insufficient magnitude to strike a discharge cell. However, if both positive-going and negative-going pulses are applied simultaneously to the row and column conductors defining one or more of the discharge cells, the combined voltage will cause the selected cell or cells to strike. When the transfer discharge moves to another set of electrodes, the discharge cells already struck will be maintained by a voltage applied by the row drive circuit.

As the transfer discharge moves along the pairs of electrodes, each column of discharge cells is scanned in sequence, and a display may be produced by energising the appropriate row drive circuits at the required time. It is only necessary to scan the array of discharge cells once in order to produce a display, though the scan may be continuous and repetitive if preferred.

It is possible to use the same configuration to provide for selective erasure of parts of a display, that is for extinction of one or more cells already struck by the mechanism described above. The only changes required are to arrange that each row drive circuit RD is able to produce a negative-going pulse relative to the maintaining voltage, and that the pulse circuit 10 is able to produce at the outputs W, X, Y and Z positivegoing pulses relative to a positive datum +V1. In such a case, operation of the glow transfer device would cause column conductor CC to be connected to a positive voltage, thus causing the voltage on that conductor to rise due to the voltage drop across the resistor R1. The combined effect of a positive-going pulse on a column conductor and a negative-going pulse on an anode conductor is to extinguish a struck discharge cell by reducing the voltage across it to a value below the cellextinction voltage.

When the device is intended solely for displaying alphanumeric information, characters are restricted to begin six columns apart, that is, each character is five columns wide and followed by a normally empty column separating adjacent characters. In certain circumstances, for instance while periodically changing labelled information is being displayed, it may be desired to retain the label rather than rewrite it each time that the information is changed. In the display device thus far described, fresh information may only be written once during each scan of the glow transfer tube and if a large part of the information is to be repeated a large proportion of the scan time is wasted. To overcome this delay, reset conductors similar to RS1 and RS2 may be added to the glow transfer tube at a number of points along its length.

While it is impracticable to arrange a display device of, say, 210 columns so that the scan may begin and end at any particular column, there will only be 35 a1- phanumeric characters on the line so that it is possible to have a pair of reset conductors before each character. For a larger display device of, say 1,000 columns, itis possible to have reset conductors for groups of live alphanumeric characters thus still permitting a much shorter scan time for the addition of relatively few extra components.

Although, as stated above, there is a normally empty column separating adjacent characters, this column is.

frequently used to display decimal points or punctuation marks between the characters and it is not possible to use the transfer conductors of the column as reset conductors. Reset conductors may be formed by inserting an extra pair of conductors after every six transfer conductors or by having two connections to every sixth transfer conductors as shown in FIG. 2.

The conductor TC6 is connected to the three phase supply by way of a diode D1 which allows the negativegoing glow transfer pulse Z (V2) to be applied to the conductor in the normal way. The other connection is from a reset circuit W1 similar to the output W of the pulse circuit 10. Thus because of the diode D1, a large reset voltage (V3) may be applied to the transfer conductor TC4 without being simultaneously applied to the other transfer conductors on the same phase. Also wherever the glow is initiated, the successive transfer conductors will operate normally.

The various voltages necessary to operate a display device as described will, of course, depend upon the physical dimensions of the device. A typical display matrix has a striking voltage of 400 volts, and a maintaining voltage of 300 volts. It is therefore possible to arrange for each row drive circuit to deliver a DC. output of +300 volts with a superimposed 60 volt pulse when writing onto the display,-and a 60 volt pulse when erasing a display. The current flow through the glow transfer discharge when writing is such that the voltage drop across a column resistor R1 is 50 volts, thus giving the necessary 100 volts increase over the maintaining voltage to strike a discharge cell. The glow transfer device may typically have a striking voltage of 250 volts and an extinguishing voltage of 180 volts. Hence to form the discharge between the reset electrodes the output W of the pulse circuit must be -250 (-V3) volts, and must subsequently rise to a value not greater than l80 volts (.V1) to extinguish the discharge. The other outputs X, Y, and Z should vary between -l80 volts (VI) and, say, 240 volts (-V2), the latter value being sufficient to cause the discharge to transfer but not sufficient. to strike a new discharge. For erasing a display, the polarities of the outputs W, X, Y and Z should be reversed. The above are only examples, and should not be taken as limiting values.

FIGS. 4 and 5 illustrate the construction of a display device of the type described above. Referring to FIGS. 4 and 5, a display device according to the invention is built up around a spacer plate 12. This is a block of an electrically-insulating material such as glass or ceramic having formed in it a matrix of small cavities 13. These cavities may be of circular or square cross-section, and are arranged in the rows and columns of a rectangular matrix. Secured to one surface of the spacer plate is a block 14, of similar material, carrying a plurality of parallel anode conductors 15, say of stannic oxide. Each anode conductor passes between the cavities 13 in each row of the matrix, which is intended to be viewed from the top as indicated. Each discharge cell formed by a cavity is provided with a transparent anode electrode 16, also formed on the block 14, and each anode electrode is connected to a selected one of the anode (or row) conductors 15 by means of a layer 17 of electrically-resistive material. This layer may have a resistance of the order of a few megohms, and forms the series resistance of the discharge cell.

The lower end of each discharge cell has a cathode electrode, the electrodes of all the cells in a column of the matrix being formed by a cathode conductor 18. Hence the underside of the spacer plate 12 carries a plurality of parallel cathode (or column) conductors l8. Separated from the spacer plate 12 by a spacer 19 is a further block 20 of electrically-insulating material. This carries a plurality of transfer conductors 21 each parallel to and opposite one of the cathode conductors 18. In addition, the spacer plate 12 carries a conductor 22 and the block 20 carries an opposite conductor 23. The two conductors 22 and 23 form the reset electrodes of the glow transfer device, whilst the opposite pairs of cathode conductors 18 and conductors 21 form the pairs of electrodes of the glow transfer device.

The space between the block 20 and spacer plate 12, and the cavities 13 in the spacer plate are filled with a suitable gas when the joints between the parts of the device have been sealed.

The formation of the various conductors, resistors and electrodes requires a number of operations, and the components may be made from a number of alternative materials. The series resistors 17 may be formed by a screen printing process, using either ruthenium dioxide or thallium oxide. After printing, the resistors are fired, at about 650 C for ruthenium dioxide and at about 500 C for thallium oxide-The anode conductors and electrodes may be formed by evaporating stannic oxide through a mask. 5 The cathode conductors, which also form the electrodes, have to pass over the ends of the cavities in the spacer block, and hence it is not possible to use evaporation or deposition techniques. The cathode conductors may be formed from thin metal strips secured to the underside of the spacer plate. The material of the strips has to be one which may be readily sealed to the glass block, and materials such as the nickelchromium-iron alloys are suitable. The transfer conductors may be formed by metal strips or by evaporation and plating. An evaporated layer of chromium covered with a layer of nickel is suitable.

Other forms of construction and other materials for the various parts may also be used.

The transfer conductors, each forming one electrode of a pair in the glow-transfer device, are of large area to ensure that the impedance of the current path through the glow transfer device is low, irrespective of the current.

The embodiment described above uses a glow transfer device which is integral with the matrix of discharge cells. As an alternative it is possible to use an external glow transfer device, electrically connected to the matrix in the same manner. Such a glow transfer device would not require multiple anode and cathode electrodes is the usual glow-transfer mechanism by means of guide electrodes was employed. For a large display device having, say, a thousand columns of discharge cells, the integral form of glow transfer device would probably be essential.

It will be seen from the above description that the invention allows a very considerable saving in drive circuitry. Only four drive circuits are required, irrespective of the number of columns of discharge cells, to drive all the columns in the matrix, though each row still requires a separate drive circuit.

I claim:

1. A visual display device which includes a plurality of discharge cells comprising a block of electrically insulating material containing a plurality of gas-filled cavities arranged in a two co-ordinate array, a first set of electrical conductors located adjacent like ends of the cavities and in contact with the discharge gas therein, each conductor of said first set forming first electrodes of a particular group of cells extending along one coordinate of the array, a plurality of second electrodes each located adjacent the other end of an individual cavity and in contact with the discharge gas therein, a plurality of electrically resistive elements each of which is connected to a different second electrode, a second set of electrical conductors, each conductor of said second set interconnecting the resistive elements of a particular group of cells extending along the other coordinate of the array, and a glow transfer device including a plurality of pairs of spaced electrodes in a common gas-filled discharge space, one electrode of each of the pairs comprising a conductor of one of said sets of electrical conductors and the other electrode of each of the pairs extending parallel to said one electrode, and means for sequentially energising said other electrodes of the pairs of electrodes so as to cause a discharge formed between the electrodes of each pair to scan along the glow transfer device by the mechanism of glow transfer and to change the potential of each of said one electrodes of the pairs of electrodes in turn.

2. A visual display device as claimed in claim 1 in which said one electrode of each of said pairs of spaced electrodes is one of the first set of electrical conductors.

3. A visual display device as claimed in claim 2 in which the first set of electrical conductors are formed on the block of electrically insulating material.

4. A visual display device as claimed in claim'3 in which said one electrode of each pair of spaced electrodes is formed on a further block of electrically insulating material, spaced from the block containing the array of cavities.

5. A visual display device as claimed in claim 1 in which the glow transfer device has at least one pair of glow initiating electrodes.

6. A visual display device as claimed in claim in which said one pair of glow initiating electrodes are ad- 5 jacent the first pair of electrodes of the glow transfer electrodes.

5 3 UNITED STATES PATENT OFFICE v CERTIFICATE OF CQRRECTION Dated January 22, 1974 Patent No. 3 787, 753

Invent r) Frank Walters It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

[- Cover' page, aiter item '[2l] insert: 1

--[30] Foreign Application Priority Data August 1.2, 1971 Great Britain '38826/7l--.

Signed and sealed this llth day r June 1971;.

Attest:

EDWARD mmnrrcntmm. I c; MARSHALL DANN Attesting Of1cer Commissioner of Patents 

1. A visual display device which includes a plurality of discharge cells comprising a block of electrically insulating material containing a plurality of gas-filled cavities arranged in a two co-ordinate array, a first set of electrical conductors located adjacent like ends of the cavities and in contact with the discharge gas therein, each conductor of said first set forming first electrodes of a particular group of cells extending along one coordinate of the array, a plurality of second electrodes each located adjacent the other end of an individual cavity and in contact with the discharge gas therein, a plurality of electrically resistive elements each of which is connected to a different second electrode, a second set of electrical conductors, each conductor of said second set interconnecting the resistive elements of a particular group of cells extending along the other co-ordinate of the array, and a glow transfer device including a plurality of pairs of spaced electrodes in a common gas-filled discharge space, one electrode of each of the pairs comprising a conductor of one of said sets of electrical conductors and the other electrode of each of the pairs extending parallel to said one electrode, and means for sequentially energising said other electrodes of the pairs of electrodes so as to cause a discharge formed between the electrodes of each pair to scan along the glow transfer device by the mechanism of glow transfer and to change the potential of each of said one electrodes of the pairs of electrodes in turn.
 2. A visual display device as claimed in claim 1 in which said one electrode of each of said pairs of spaced electrodes is one of the first set of electrical conductors.
 3. A visual display device as claimed in claim 2 in which the first set of electrical conductors are formed on the block of electrically insulating material.
 4. A visual display device as claimed in claim 3 in which said one electrode of each pair of spaced electrodes is formed on a further block of electrically insulating material, spaced from the block containing the array of cavities.
 5. A visual display device as claimed in claim 1 in which the glow transfer device has at least one pair of glow initiating electrodes.
 6. A visual display device as claimed in claim 5 in which said one pair of glow initiating electrodes are adjacent the first pair of electrodes of the glow transfer device.
 7. A visual display device as claimed in claim 5 in which a plurality of pairs of glow initiating electrodes are dispersed throughout the glow transfer device.
 8. A visual display device as claimed in claim 5 in which selected pairs of electrodes of the glow transfer device are also adapted to be used as glow initiating electrodes. 